1. Field of the Invention
This invention relates to an improvement in a catalytic cracking process. More particularly, it relates to stripping entrained nitrogen from regenerated cracking catalyst.
2. Description of the Prior Art
Catalytic cracking processes in which a hydrocarbonaceous oil is converted to lower boiling hydrocarbons in the presence of a cracking catalyst are well known (see, for example, Hydrocarbon Processing, September 1976, pages 114 to 120). Generally, at least a portion of the spent cracking catalyst is removed from the cracking zone and passed to a regeneration zone where the catalyst is contacted with an oxygen-containing gas such as air at an elevated temperature to remove the carbonaceous deposit from the catalyst and thereby regenerate the catalyst. Usually, prior to passing the spent catalyst to the regeneration zone, the spent catalyst is contacted with a stripping gas to remove adhering or entrained hydrocarbons from the catalyst. After regeneration, the regenerated catalyst is recycled to the catalytic cracking zone. When the oxygen-containing gas used in the catalyst regeneration zone is air, the regenerated catalyst particles recovered from the regeneration zone include entrained nitrogen. Recycling of the regenerated catalyst particles including the entrained nitrogen to the catalytic cracking zone in turn causes the catalytic cracking zone effluent to comprise nitrogen. Typically, the effluent of the catalytic cracking zone is separated into a normally gaseous product and a normally liquid product. The normally gaseous product comprises the C.sub.2 -- gases (components boiling not higher than the two carbon atom-containing hydrocarbons), that is, CH.sub.4, C.sub.2 H.sub.4, C.sub.2 H.sub.6, H.sub.2 and, as contaminants, N.sub.2, carbon oxides and hydrogen sulfide. All the contaminants, except nitrogen, can readily be recovered from the C.sub.2 -- gaseous stream by conventional means, such as acid scrubbing and cryogenic separation. Nitrogen, however, cannot be readily recovered. Therefore, the C.sub.2 -- stream is usually burned rather than being further processed.
It is known to strip a regenerated catalyst which contains entrained nitrogen with a stripping gas such as steam (see, for example, U.S. Pat. No. 3,923,686). However, steam has the disadvantage of rapidly deactivating the regenerated catalyst.
It is also known to use steam or flue gas for stripping oxygen-containing gases from a completely regenerated catalyst. Flue gas, however, contains an appreciable amount of nitrogen and is therefore not efficient for stripping nitrogen.
It has now been found that if nitrogen is stripped from the regenerated catalyst particles, with a CO.sub.2 -containing gas, under specified conditions, then the normally gaseous product of a catalytic cracking zone effluent is not contaminated by nitrogen and the C.sub.2 -- stream can be used in various processes or the hydrogen component can be recovered from the C.sub.2 -- stream for use, if desired, in hydrotreating processes.